1. Field of the Invention
The present invention relates to a cassette for use in radiation image forming apparatus for storing a radiation image recording medium on which to record radiation image information of a subject.
2. Description of the Related Art
There is known a system for recording radiation image information of a subject such as a human body with a stimulable phosphor, and reproducing the recorded radiation image information on a photosensitive medium such as a photographic film, or displaying the recorded radiation image information on a display unit such as a CRT or the like.
The stimulable phosphor is a phosphor which, when exposed to an applied radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores a part of the energy of the radiation, and, when subsequently exposed to applied stimulating rays such as visible light, emits light in proportion to the stored energy of the radiation. Usually, a sheet provided with a layer of the stimulable phosphor is used as a stimulable phosphor sheet.
It has been practiced to apply a radiation, e.g., X-rays, to a subject such as a human body, to record radiation image information of the subject directly on a photographic film. The radiation image information recorded on the photographic film is subsequently developed into a visible image, which will be used for a medical diagnosis or the like.
Each radiation image recording medium such as a stimulable phosphor sheet or a photographic film is usually housed in a cassette and loaded into an exposure apparatus where X-rays are applied to the radiation image recording medium through the cassette. Known cassettes of this type are disclosed in Japanese Laid-Open Patent Publication No. 2003-207864, Japanese Laid-Open Patent Publication No. 2003-270747, and Japanese Laid-Open Patent Publication No. 6-19021, for example.
FIG. 13 of the accompanying drawings shows in exploded perspective a mammographic cassette 100 to be placed along the chest wall of a subject, as disclosed in Japanese Laid-Open Patent Publication No. 2003-207864 and Japanese Laid-Open Patent Publication No. 2003-270747. The mammographic cassette 100 has a front plate 102 and a back plate 104. A stimulable phosphor sheet 106 is placed on the surface of the back plate 104 which faces the front plate 102. In FIG. 13, the mammographic cassette 100 is illustrated upside down.
FIG. 14 of the accompanying drawings shows in perspective the interior of the mammographic cassette 100 that is completed when the front plate 102 and the back plate 104 are combined with each other. In FIG. 14, the bottom of the back plate 104, which is opposite to the stimulable phosphor sheet 106, is omitted from illustration.
In the back plate 104, there are disposed a push latch 108, two springs 110a, 110b fixing the push latch 108 to the back plate 104 and normally biasing the push latch 108 in the direction indicated by the arrow Q1, a substantially U-shaped joint member 114 fixed to the back plate 104 by two springs 112a, 112b which normally bias the joint member 114 in the direction indicated by the arrow Q1, and four lock fingers 116, 118, 120, 122 connected to the joint member 114.
The push latch 108 houses a moving mechanism (not shown) therein. Each time a rod 136 is pressed, from outside of the cassette 100, against the push latch 108 in the direction indicated by the arrow Q2, the moving mechanism is actuated to move the push latch 108 in the direction indicated by the arrow Q1 or in the direction indicated by the arrow Q2.
In FIG. 14, the push latch 108 is shown as being displaced in the direction indicated by the arrow Q2 against the bias of the springs 110a, 110b in the direction indicated by the arrow Q1, and the joint member 114 is shown as being displaced in the direction indicated by the arrow Q2 against the bias of the springs 112a, 112b. 
For putting the stimulable phosphor sheet 106 into the cassette 100, as shown in FIG. 13, the stimulable phosphor sheet 106 is placed on the surface of the back plate 104, and the front plate 102 and the back plate 104 are superposed on each other sandwiching the stimulable phosphor sheet 106.
Then, as shown in FIG. 14, the rod 136 is pressed, from outside of the cassette 100, against the push latch 108, actuating the moving mechanism. The push latch 108 is now displaced in the direction indicated by the arrow Q1 by the moving mechanism and under the bias of the springs 110a, 110b. The joint member 114 and the lock fingers 116 through 122 are released from the push imparted by the push latch 108 in the direction indicated by the arrow Q2, and displaced in the direction indicated by the arrow Q1 under the bias of the springs 112a, 112b. 
The lock fingers 120, 122 engage respective link members 126, 128 that are connected to both ends of a frame 124 of the front plate 102, respectively. Therefore, the link members 126, 128 are also displaced in the direction indicated by the arrow Q1.
As shown in FIG. 15, the frame. 124 is turned in the direction indicated by the arrow Q3, causing a tip end 130 of the frame 124 to engage in a groove 132 in the back plate 104. The interior of the cassette 100 is now shielded from entry of light. The stimulable phosphor sheet 106 has an end face 134 held in abutment against an inner surface of the frame 124, and is held within the cassette 100.
The cassette 100 with the stimulable phosphor sheet 106 housed therein is then loaded into an exposure apparatus such that the frame 124 is placed against the chest wall of a subject. Then, radiation image information of the subject is recorded on the stimulable phosphor sheet 106 through the cassette 100.
FIG. 16 of the accompanying drawings shows a cassette 140 disclosed in Japanese Laid-Open Patent Publication No. 6-19021. As shown in FIG. 16, the cassette 140 has a base plate 144 supporting on its upper surface a covering layer 142 made of a light-stimulus emission material and a cap 146 which sandwiches the covering layer 142 in coaction with the base plate 144.
The base plate 144 has side faces 148 slanted from its upper surface toward its bottom surface and having holes 150 defined therein. The holes 150 house therein respective springs 152 secured to the base plate 144 and connected to respective retainer pins 154 which are slidably fitted in the holes 150 and project partly out from the holes 150.
The cap 146 has a top plate 156 facing the upper surface of the base plate 144 and side plates 158 extending from the ends of the top plate 156 obliquely downwardly facing the side faces 148 of the base plate 144. A cushion layer 160 is mounted on the lower surface of the top plate 156 facing the covering layer 142. The side plates 158 of the cap 146 have respective holes 162 defined therein for receiving the respective retainer pins 154.
When the cap 146 is lowered toward the covering layer 142 placed on the upper surface of the base plate 144, the side plates 158 of the cap 146 press the retainer pins 154 into the respective holes 150. Upon continued descent of the cap 146, the holes 162 are brought into alignment with the respective holes 150, whereupon the retainer pins 154 move from the holes 150 into the holes 162 under the bias of the springs 152. The cap 146 causes the cushion layer 160 to press the covering layer 142 on the base plate 144. The covering layer 142 is now retained in the cassette 140 by the retainer pins 154 which engage both the base plate 144 and the cap 146.
According to Japanese Laid-Open Patent Publication No. 2003-207864 and Japanese Laid-Open Patent Publication No. 2003-270747, the stimulable phosphor sheet 106 is simply placed in abutment against the frame 124 in the cassette 100. Therefore, the stimulable phosphor sheet 106 tends to be easily displaced out of position when the cassette 100 is vibrated.
The back plate 104 of the cassette 100 accommodates a number of parts, such as the springs 110a, 110b, 112a, 112b, to keep the stimulable phosphor sheet 106 as closely to the chest wall of the subject as possible. Accordingly, the cassette 100 is structurally complex and highly costly to manufacture.
According to Japanese Laid-Open Patent Publication No. 6-19021, the covering layer 142 is pressed simply by the cap 146 through the cushion layer 160. Therefore, the covering layer 142 is also liable to be easily displaced out of position upon vibration of the cassette 140. In addition, as the cassette 140 is made up of a number of parts such as the springs 152, the retainer pins 154, and the cushion layer 160, the cassette 140 is of a complex structure and is highly expensive to manufacture.